Abstract : The structures of various boundaries and interphase interfaces are viewed as the alignment of lattice plane networks in the two crystals, such that 'optimum matching' is achieved at the interface. A large variety of planar boundaries, including rather complex mixed tilt and twist boundaries, may be viewed in this way. Congruent networks correspond with exact coincidence boundaries; commensurate networks correspond with near-coincidence boundaries. The nature and form of the arrays of grain boundary dislocations resulting from different types of misalignment of the networks are discussed. Secondary dislocations which accommodate rotational mismatch, and registry dislocations which accommodate lattice mismatch, are both shown to be related to network misalignment. Finally, the lattice plane view of various structural unit models of grain boundaries is presented. (Author)